Heat generating structures

ABSTRACT

A heat generating structure includes a substrate of a first material and a second material coating at least a portion and preferably all of the first material, where the second material is different from the first material. The structure also includes an additional material or compound such as ammonia borane that is impregnated or located within the structure. When the structure is thermally energized, the first and second materials react with each other in an exothermic and self-sustaining reaction that pyrolyzes the impregnated ammonia borane compound to create a target gas, for example, hydrogen from the ammonia borane. An additional material, for example, a thermite, may be interposed between the structure and the ammonia borane to facilitate the ignition of the ammonia borane.

GOVERNMENT RIGHTS

This invention was made with Government support under contract W909MY-06-C-0041 awarded by the U.S. Army. The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

The invention relates in general to heat generating structures, and more particularly to a heat generating structure that provides heat for various purposes, including pyrolysis of various materials or compounds impregnated or located within the structure to generate a desired or target gas.

Fuel cells require hydrogen for proper operation. One proposed source of hydrogen is through the pyrolysis of ammonia borane using a heat source in pellet form. This net exothermic process involving ammonia borane is useful as a source of hydrogen because of its relatively high hydrogen content—ammonia borane is approximately twenty percent by weight hydrogen. Typically, heating of the ammonia borane occurs at a steady rate to achieve the desired pyrolytic decomposition temperature of several hundred degrees Centigrade. Practical commercial applications of such technology require both high utilization efficiency and economic efficiency (relatively high ammonia borane content per system). Efforts to pyrolyze ammonia borane contained within pressed pellets, in which ammonia borane is mixed with a type of pyrotechnic composition or thermite composed of, for example, aluminum, boron and ferric oxide powders. At high thermite content (e.g., 80%) of the pellet, gravimetric hydrogen generation is relatively low because of the correspondingly low ammonia borane pellet content (e.g., 20%). In contrast, at relatively high ammonia borane content, the resulting amount of hydrogen generated is also low because the driving thermite reaction cannot be sustained and the exothermicity of the ammonia borane pyrolysis is insufficient to continue its own decomposition—that is, the thermite concentration in the pellet is too “dilute” to completely propagate throughout the pellet Also, the thermite is typically of a particulate nature which may lead to propagation problems irrespective of its content (i.e., discontinuous or shorter than desired propagation duration, particle size variations, mixing inconsistencies and/or compaction variations) because the thermite is not physically continuous.

What is needed is a continuous heat generating structure composed, for example, of two dissimilar materials, such as metals, that provide sufficient heat for various purposes, including the complete pyrolysis of a compound (e.g., ammonia borane) impregnated or located within the structure, where the structure has a relatively high content of the compound for the generation of a correspondingly high amount of a target gas (e.g., hydrogen from pyrolysis of ammonia borane), where the exothermic alloying reaction occurring within the dissimilar metals of the heat generating structure propagates largely independent of the compound in contact with the structure, and where the only gases given off are those of the pyrolysis reaction products, for example, hydrogen.

SUMMARY OF THE INVENTION

Briefly, according to as aspect of the present invention, a heat generating structure includes a substrate comprised of a first material and a second material coating at least a portion and preferably all of the first material, where the second material is different from the first material. The structure also includes a material or compound such as ammonia borane that is impregnated or located within the structure. When the structure is thermally energized, the first and second materials react with each other in an exothermic and self-sustaining reaction that pyrolyzes the impregnated compound to create a target gas, for example, hydrogen from the ammonia borane. An additional material, for example, a thermite, may be interposed between the structure and the ammonia borane to facilitate the ignition of the ammonia borane.

These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a mesh substrate having a plurality of wires each coated with a material in an embodiment of a heat generating structure of the present invention;

FIG. 2 is a cross-section of one of the wires in the mesh substrate of FIG. 1;

FIG. 3 is a cross-section of the mesh substrate of FIG. 1 interposed between various other layers, including layers of a compound that is intended to be pyrolyzed by the heat generating structure of the present invention;

FIG. 4 is a side view of an alternative embodiment of a foam substrate of the heat generating structure of the present invention;

FIG. 5 is a sectional view of the foam substrate of FIG. 4 located on another substrate; and

FIG. 6 is a cross-section of an alternative embodiment of a foil substrate of the heat generating structure of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there illustrated is a plan view of a preferred embodiment of a heat generating structure 10 in accordance with the present invention. The structure 10 may comprise at least two constituent portions: a substrate 12 comprised of a plurality of wires 14 of one material, preferably a reactive metal, where the substrate 12 is preferably continuous in all three dimensions; and a coating 16 of a second material, preferably also a reactive metal that is different from the metal comprising the wires 14. The coating 16 is applied on at least a portion of each substrate wire 14 and preferably, on the entirety of each substrate wire 14, to thereby form a substrate 12 of continuously-coated wires 14. Preferably, the three-dimensional structure of the substrate 12 is such that it has an appreciable amount of free volume (i.e., empty space creating voids 18 between crossing wires 14 in the mesh substrate 12). Thus, in FIG. 1 the substrate 12 is preferably a continuous mesh structure that comprises a plurality of intersecting straight metal wires 14 with voids or empty spaces 18 located between intersecting wires 14. The wires 14 are preferably in intimate physical and, thus, in thermal contact with one another at the intersections 20. In an exemplary preferred embodiment, the mesh substrate 12 is commercially available from TWP, Inc. of Berkeley, Calif., and comprises a plurality of aluminum wires 14 each with an approximate thickness or diameter in the range of from 0.0021 inches (200 wires per inch) to 0.0090 inches (40 wires per inch).

In a preferred embodiment, the coating 16 comprises nickel that is applied onto the outer surface of each of the wires 14 of the aluminum metal mesh substrate 12 by, for example, electroplating or other methods such as vacuum sputtering or through an electroless process. The nickel coating 16 may include other materials including boron, phosphorus and/or palladium. Also, if aluminum is utilized as the mesh substrate material, any aluminum oxide that is present on the outer surface of the aluminum wires 14 may be removed and a coating of zinc may be applied to the outer surface of the wires 14. The zinc may allow initiation or ignition of the structure 10 at a lower temperature than if the zinc were not present. In the alternative, the zinc coating may be removed if an electroless process is used to coat the nickel onto the aluminum wire 14. The amount of nickel that is coated onto the aluminum mesh wires 14 may be in a one to one ratio with the underlying aluminum wires 14; that is, the nickel may be in an equivalent molar content as that of the aluminum. Such a ratio of nickel to aluminum allows for the continuous self propagation in an exothermic heat evolution reaction when the mesh substrate 12 is ignited, as described in detail hereinafter. The heat generating structure 10 may be considered to be a reactive multilayer laminate comprising the substrate 12 and the coating 16, with both the substrate 12 and the coating 16 comprising reactive metals in a preferred embodiment.

In a variation of the substrate 12 of the heat generating structure 10 of the present invention, the substrate 12 may simply comprise a single wire 14 comprising one material that is coated with a second material.

FIG. 2 illustrates a cross section of one of the aluminum wires 14 in the mesh with a nickel coating 16 in a one to one atom ratio. Here, the approximate thickness of the nickel coating 16 may be 600 micro inches. If a combination of nickel and palladium is coated onto the aluminum wires 14 in the mesh substrate 12, the approximate thickness of the nickel coating 16 may be 100 micro inches and that of the palladium may be 300 micro inches. In the alternative, the ratio between the aluminum and nickel may be something other than one to one, for example, a ratio of one to three. In general, the thickness of the coating 16 is selected so as to control the alloying stoichiometry and, thus, the rate of heat evolution and propagation.

The materials (e.g., metals) comprising the substrate 12 and the coating 16 are selected based on their individual characteristics, such as melting point and density, and in combination for enthalpy of alloying. For any bimetallic structure comprising a substrate of a first metal coated with a second, different metal to propagate, the formation of alloys from the individual metal constituent components must be exothermic. This heat evolved warms not only the surrounding environment, but also the continuous metal structure. After a source of ignition (e.g., a match or a heating element such as a semiconductor bridge) is applied to the structure 10 of the present invention, the alloying temperature of the metals (typically close to the melt point of the aluminum wire 14 first) is eventually achieved and the materials are thermally energized and react with each other such that further alloying between the two metals is induced. Accordingly, heat is liberated with resulting propagation in a self-sustaining manner throughout the entire continuous heat generating structure 10 from a first or starting point within the structure 10 and along a travel path to a second or ending or discharge point within the structure 10, and preferably in a controlled and repeatably manufacturable manner. The starting and ending points are typically spaced from each other with the travel path in between.

For example, if the structure 10 is of a three-dimensional, rectangular-shape, once ignited at a first or starting end of the structure 10, the thermal energization of the reactive materials comprising the structure 10 will cause the propagation to continue through to the second or discharge end at a consistent timed rate depending on the intermetallic or bimetallic (or non-metallic) composition of the structure 10 as well as on the geometric configuration (e.g., thickness of wires 14, wire crossing frequency) of the structure 10. Thus, by controlling the composition and the configuration of the reactive materials comprising the heat generating structure 10, the burn or propagation rate can be controlled (that is, the reaction rate or time period for propagation from the first end to the second end along the travel path of the reactive material can be selected).

More specifically, the exothermic reaction between the dissimilar materials comprising the heat generating structure 10 can be made to occur at a relatively slower propagation or burn rate than prior art devices in part not only due to the composition of the dissimilar materials selected but also due to the three-dimensional characteristics of the substrate portion 12 of the structure 10; in particular, to a non-uniform and varying distribution of the mass of the substrate and corresponding coating along the direction of the primary propagation or burn path. For example, if the coated mesh structure 10 of FIG. 1 were ignited by initially igniting the seven vertical wires 14 illustrated at the top of FIG. 1 and then moving downward (along the primary burn path), the burning or propagation of these wires would continue downward at a relatively fast rate until the first horizontal cross wire 14 was encountered by each of the vertical wires 14. At these intersection points 20, the rate of propagation would slow somewhat due to the relatively greater mass encountered by the horizontal and vertical crossing wires 14 which are preferably in close physical and, thus, thermal contact with one another. Burning and propagation would travel along this horizontal wire 14 in addition to continuing to travel downward in a vertical direction in FIG. 1. Essentially, at these wire intersections 20 a non-uniform and varying distribution of mass of the substrate 12 is encountered by the burning vertical wires 14, which inherently slows down the propagation or burn rate of the entire coated wire mesh substrate 12. Once propagation has reached the first horizontal wire 14, heat liberated from the alloying of the horizontal wire 14 then accelerates propagation. The propagation rates speed up until the next horizontal wire 14 is encountered, where the propagation rate slows again. This process repeats itself until the coated wire mesh substrate 12 has been entirely burned through.

Referring to FIG. 3, in accordance with an exemplary embodiment of the present invention for use in generating heat to pyrolyze a compound to generate a target gas, there illustrated in cross-section is the heat generating structure 10 of FIG. 1 located in an larger, layered structure 22 that generates hydrogen from ammonia borane. Specifically, three separate layers of the coated wire mesh substrate 12 of the heat generating structure 10 of FIG. 1 are centrally-located within the layered structure 22. The three layers of the mesh substrate 12 may be stacked on top of each other in contact with one another or spaced a small distance apart (e.g., 2 mm) to form a multilayer laminate 24. Three layers of the mesh substrate 12 are purely exemplary; more or less layers may be utilized depending upon the desired amount of heat to be generated. Located on each side of the laminate 24 is a layer 26 of thermite, which may comprise a mixture of aluminum, boron, and iron oxide (AlBFe₂O₃) or other suitable thermite material. As illustrated in FIG. 3, since the thermite material is typically commercially available in powdered form, the thermite material may penetrate into some or all of the voids 18 (FIG. 1) of the coated mesh substrate 12 so as to partially or fully impregnate or fill these voids 18. Located next to each thermite layer 26 in the layered structure 22 is a layer 28 of ammonia borane or some other compound or material desired to be pyrolyzed into a target gas. Ammonia borane is typically available in a granulated mixture in a waxy powder form.

Depending upon the compound selected to be pyrolyzed to generate a target gas, the thermite layers 26 may or may not be needed. For example, the thermite layers 26 are required when ammonia borane is used to generate hydrogen since the propagation of the coated mesh substrates 12 forming the multilayer laminate 24 will not propagate through the ammonia borane without the thermite layers 26 located therebetween. As such, the thermite layers 26 may be considered to be an additional heat generating source, where the thermite and the coated metal mesh together pyrolyze the ammonia borane. That is, when the coated metal mesh substrates 12 are ignited and begin to propagate in a self sustaining exothermic reaction, the resulting heat generated ignites the thermite layers 26, which provide additional heat to pyrolyze the ammonia borane. If the thermite layers 26 are not utilized, then the layers 28 of the compound to be pyrolyzed may be located next to and/or within the voids of the substrate 12. Located next to each layer 28 of ammonia borane is a layer 30 of carbon foam. The carbon foam may be formed as part of the layered structure 22 such that it encases all or part of the remaining layers 24-28 of the layered structure 22, with perhaps an opening in the carbon foam for an initiation device (not shown) used to ignite the coated metal mesh substrates 12 forming the multilayer laminate 24.

In operation, when the coated metal mesh substrates 12 are ignited, the adjacent thermite layers 26 also ignite. This causes the layers 28 of ammonia borane to pyrolyze with liberation of hydrogen gas. The hydrogen gas represents the desired or target gas and permeates through the layers 30 of carbon foam or other suitable similar material and is then collected and utilized in conjunction, for example, with a fuel cell. For example, if the heat generating structure 10 comprising an aluminum mesh substrate 12 coated with an equivalent molar content of nickel is impregnated and/or located next to the thermite layers 26 and ammonia borane layers 28, the overall structure 22 can hold by weight approximately 60% ammonia borane. Thus, the heat generating structures of the present invention expands the range of structures suitable for pyrolysis of ammonia borane or other compounds or materials. This ensures a higher amount of hydrogen generated by the alloying process of the ignited heat generating structure 10. The layered structure 22 may include the layers 28 of ammonia borane using a press or a solution-based process. Through proper selection of the metals comprising the wires 14 and the coating 16, the exothermic alloying reaction of the ignited heat generating structure 10 propagates largely independent of the ammonia borane. Thus, as can be seen from the foregoing, the multilayer laminate 24 comprising the heat generating structure of the embodiment of FIG. 3 is used as a source of heat to distribute heat to other materials or compounds to pyrolyze these compounds into a target gas.

The heat generating structure 10 of the present invention is not limited to pyrolyzing ammonia borane to generate hydrogen gas. Other materials or compounds may be disposed in a structure 22 similar to that of FIG. 3 so that these materials or compounds may be pyrolyzed to generate a target gas. For example, to generate oxygen gas, the following oxygen storage compound classes may be pyrolyzed: nitrates, nitrites, chlorates, perchlorates, oxides, chromates, dichromates, permanganates, iodates, bromates, peroxides or ozonides. To generate nitrogen gas, the following nitrogen storage compound classes may be pyrolyzed: azides, nitrates, nitrites, diazos, triazolones, triazoles or hydrazines. To generate carbon dioxide gas, the following carbon dioxide storage compound classes may be pyrolyzed: carbonates, carbamates, carboxylates or dicarboxylates. To generate hydrogen gas, other compound classes besides ammonia borane that may be pyrolyzed include borohydrides and other metal hydrides, carboranes or hydrazines. With regard to the generation of oxygen, because the various oxides used during the pyrolytic decomposition in general have a relatively low thermal conductivity, the thermite layers 26 of the structure 22 of FIG. 3 may not be needed. Further, with regard to all of the various compounds listed above with respect to generation of the various target gases, additional materials may be needed and/or utilized in a structure such as the structure 22 of FIG. 3 to achieve the proper pyrolytic decomposition of the compound into the target gas. The additional materials include inorganic salts, metal complexes, and organic derivatives in a given form pure, dispersed in a binder, and/or stored as imbibed nanotubles, filled zeolite, or as an adsorbate. The specific method of usage of these additional materials should be apparent to one of ordinary skill in the art in light of the teachings herein. Also, if it is desired to modify the rate of the reaction that generates the target gas, it is possible to add a binder (as discussed in detail hereinafter), polymer and/or a catalyst to the compound that is pyrolyzed into the target gas. Further, if it desired to lower the rate of heat liberation by the materials that comprise the heat generating structure of the present invention, it is possible to add in an insulator material in, e.g., layer form, in between the mesh substrate 12 of FIG. 1 and the compound or material that is desired to be pyrolyzed. These additions to the heat generating structure of the present invention should be apparent to one of ordinary skill in the art in light of the teachings herein.

Referring to FIG. 4, there illustrated is an alternative embodiment of a heat generating structure 32 of the present invention in which the substrate 34 comprises a foamed material. As compared to the mesh substrate 12 of FIG. 1, the foam substrate 34 of FIG. 4 contains many more wires 36 going off in many more different directions at one or more instants in time. The foam substrate 34 may be that commercially available from ERG Materials and Aerospace Corp. of Oakland, Calif., and may comprise aluminum foam which includes approximately 7% by volume, with approximately forty pores per inch. The majority of the wire filaments 36 comprising the foam substrate 34 are preferably less than 0.02 inches in diameter. In an exemplary preferred embodiment, the foam substrate 34 may be electroplated or coated with an equivalent molar content of nickel, thereby reducing the free volume of the foam to approximately 88%. However, the molar content of the nickel coating 38 may be anywhere in the range of 70% to 150% of the molar content of the aluminum. The foam substrate 34 may be coated in a similar manner as that of the mesh substrate 12 of FIG. 1 described hereinabove. Also, the foam substrate 34 of the heat generating structure 32 of FIG. 4 may be located in the layered structure 22 of FIG. 3 in place of the mesh substrates 12. That is, the one or more layers of mesh substrates 12 illustrated in FIG. 3 may be replaced with one or more layers comprising the foam substrate 34.

The three-dimensional characteristics of the heat generating structure of the present invention result in the transmission of heat into a structure comprising a substrate with a network of many different burn or propagation directions at one or more instants in time, in addition to the propagation direction along the desired burn path. This is particularly true with respect to the foam substrate 34 as compared to the mesh substrate 12 in terms of the number of different burn or propagation directions at one or more instants in time. Unlike systems based on compacted powder, the continuous nature of the mesh substrate 12 or foam substrate 34 and the intimate contact between the metal coating the substrates eliminates the pressure dependency required for intimate contact between layers. Thus, the burn rate of the structure is less dependent upon pressure. Failures are also reduced as a consequence of the many filaments and intersections inherent in the substrates 12, 34. In addition, consistency is enhanced since the exothermicity is controlled by the content of the metals, which content is, in turn, characterized by easily measurable and controllable weights and thicknesses (i.e., a composition and a configuration).

In general, any material that can be prepared or formed as a foam substrate 34, a mesh substrate 12, or other solid (FIG. 6) or non-solid substrate may be used as the substrate. This includes various metals and non-metals. In a preferred embodiment, the substrate material comprises aluminum and the coating comprises nickel that is either pure or combined with boron, phosphorus and/or a layer of palladium. The material comprising the substrate is typically selected in accordance with or in dependence on the material that will be coated onto the substrate. The material coated onto the substrate is preferably deposited in a reliable and consistent manner, for example by electrochemical means such as electroplating or by an autoctalytic electroless process. The materials that may comprise the substrate wires and/or the wire coating may include those from the group of reactive metals including aluminum, boron, carbon, silicon, zirconium, iron, copper, beryllium, tungsten, hafnium, antimony, magnesium, molybdenum, zinc, tin, nickel, palladium, phosphorus, sulfur, tantalum, manganese, cobalt, chromium, and vanadium.

Also, metal particles such as aluminum, magnesium, boron, beryllium, zirconium, titanium, zinc may be used in combination with fluoropolymers such as polytetrafluoroethylene, fluoroelastomers, fluorosurfactants, or fluoroadditives. As such, the metals may be formed in finely divided particles within a matrix of one of the polymers and extruded to form a wire-like structure such as a filament which is then integrated into the structure of the substrate (e.g., the wire mesh substrate 12 of FIG. 1). Thus, instead of the coated wire illustrated in FIG. 2 comprising two different materials (e.g., metals), a cross section of a wire filament that includes metal particles within a matrix of a polymer would be continuous. In the alternative, a tube made from, e.g., aluminum, may be filled with the above noted metal particles mixed in with one of the above noted polymers. A plurality of such tubes may then be used to form the substrate. Additional materials that may be utilized include energetic polymers (i.e., energetic binders) and plasticizers such as glycidyl azide polymer, polyoxetanes, or polyglycidyl nitrate. These materials may be used alone as the substrate material or in combination with any of the above reactive metals or non-reactive metals, for example, by forming these polymers and plasticizers around the single metallic wire or bimetallic wire and then integrating these wires into the structure of the substrate (e.g., the wire mesh substrate 12 of FIG. 1). These energetic materials may be used either alone or in combination with any of the above reactive or non-reactive metals by placing them inside of an aluminum tube and having a plurality of such tubes comprise the substrate.

Further, the following non-energetic polymers (i.e., non-energetic binders) can be combined with any of the above materials to form the substrate: hydroxy terminated polybutadiene, hydroxy terminated polyether, carboxy terminated polybutadiene, polyether, polycaprolactone, or polyvinyl chloride. Alternatively, such a combination of non-energetic polymers can be placed inside of an aluminum tube, where a plurality of such tubes comprises the substrate. In addition, there exist many powder-based reactions composed of a fuel and an oxidizer that constitute the bulk of pyrotechnic formulations, such as a thermite. Incorporating these into the heat generating structure of the present invention necessitates the use of a binder material, such as the energetic polymers and plasticizers listed above or the non-energetic polymers listed above, together with a non-reacting metal wire material. Thus, referring to FIG. 2, in the alternative the coating 16 may comprise a pyrotechnic formulation together with an energetic binder or a non-energetic binder. Also, a non-energetic binder may be used with the coated wire of FIG. 2 that comprises two different reactive metals such that the non-energetic binder coats at least the first and second metals and adheres the first and second metals together.

Referring to FIG. 5, in accordance with yet another embodiment of the heat generating structure of the present invention, the mesh substrate 12 of FIG. 1 or the foam substrate 34 of FIG. 4 may include a second substrate 40 that is in contact with the mesh or foam substrate of the associated heat generating structure. In FIG. 5, the foam substrate 34 is illustrated as being applied on top of or, in the alternative, within, the second substrate 40. The second substrate 40 may comprise, for example, an insulating material that is utilized to retain the heat generated during the alloying process. The insulating material of the second substrate may comprise a ceramic material or other material such as diatomaceous earth, silica, Aero-sil commercially available from Degussa in Germany, Cabosil commercially available from Cabot Corp. of Billerica, Mass., or a foamed ceramic commercially available from Aspen Aerogels, Inc. of Northborough, Mass., or argon. In the alternative, the second substrate may comprise a conductive material.

Referring to FIG. 6, there illustrated is an alternative embodiment of a heat generating structure 42 that comprises a substrate 44 that includes a plurality of layers 46 of a solid material. In this embodiment, each layer 46 preferably is identical and comprises a mixture of aluminum powder and Teflon particles that are pressed together to form the corresponding layer 46. The layers 46 may then be stacked on top of each other to form the multilayer laminate substrate 44. Thus, in this embodiment the substrate 44 differs from the substrates 12, 34 in the embodiments in FIGS. 1 and 4 in that the substrate 44 is solid throughout and contains no voids 18, as in the mesh substrate of FIG. 1 or the foam substrate 34 of FIG. 4. The solid substrate 44 of FIG. 6 may be utilized in the layered structure 22 of FIG. 3 in place of the mesh substrates 12 or the foam substrates 34. Also, the solid substrate 44 of FIG. 6 may be utilized with the second substrate 40 in the embodiment of FIG. 5. In the alternative, the aluminum powder and Teflon particles may be formed in the nature of a wire, and a plurality of such wires may be formed in a mesh such as the mesh substrate 12 of the heat generating structure of FIG. 1.

The present invention has been described and illustrated herein in conjunction with various preferred embodiments as comprising a heat generating structure that is used to pyrolyze another material to produce a target gas. However, the present invention is not limited as such. The present invention may comprise a heat source that is used for to provide heat for various purposes or applications other than for the pyrolysis of another material or compound. For example, the present invention may be used to provide localized heat to certain areas in a relatively precise manner.

Although the present invention has been illustrated and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention. 

1. Apparatus, comprising: a heat generating structure comprised of a substrate of a first material and a second material coating at least a portion of the first material, where the second material is different from the first material; and a third material located next to or within the heat generating structure, where the first and second materials, upon being thermally energized, react with each other in an exothermic and self-sustaining reaction that propagates from a first location within the structure along a travel path to a second location within the structure and at a rate that depends upon one or more characteristics of the substrate and the coating, and where the exothermic and self-sustaining reaction of the first and second materials pyrolyzes the third material.
 2. The apparatus of claim 1, further comprising a fourth material located between the substrate and the third material, where the fourth material comprises one of a pyrotechnic material or an insulator material.
 3. The apparatus of claim 2, where the pyrotechnic material comprises a thermite.
 4. The apparatus of claim 1, where at least a portion of the substrate is from the group that comprises a foam and a mesh.
 5. The apparatus of claim 1, further comprising a second substrate, where the substrate is located on or within the second substrate.
 6. The apparatus of claim 1, where the first and/or second materials are from the group that comprises aluminum, boron, carbon, silicon, zirconium, iron, copper, beryllium, tungsten, hafnium, antimony, magnesium, molybdenum, zinc, tin, nickel, palladium, phosphorus, sulfur, tantalum, manganese, cobalt, chromium, or vanadium.
 7. The apparatus of claim 1, where the one or more characteristics of the substrate and the coating include a composition of the first and second materials and a physical configuration of the substrate and the coating, and where the physical configuration of the substrate and the coating includes one or more different directions of the first and second materials at one or more points along the travel path.
 8. The apparatus of claim 1, where the first and/or second materials further comprise an energetic binder from the group that comprises glycidyl azide polymer, polyoxetanes, or polyglycidyl nitrate.
 9. The apparatus of claim 1, where the first and/or second materials further comprise a binder from the group that comprises hydroxy terminated polybutadiene, hydroxy terminated polyether, carboxy terminated polybutadiene, polyether, polycaprolactone, or polyvinyl chloride.
 10. The apparatus of claim 1, where the third material is from the group that comprises nitrates, nitrites, chlorates, perchlorates, oxides, chromates, dichromates, permanganates, iodates, bromates, peroxides or ozonides, where the third material when pyrolyzed by the exothermic and self-sustaining reaction of the first and second materials produces oxygen gas.
 11. The apparatus of claim 1, where the third material is from the group that comprises azides, nitrates, nitrites, diazos, triazolones, triazoles or hydrazines, where the third material when pyrolyzed by the exothermic and self-sustaining reaction of the first and second materials produces nitrogen gas.
 12. The apparatus of claim 1, where the third material is from the group that comprises carbonates, carbamates, carboxylates or dicarboxylates, where the third material when pyrolyzed by the exothermic and self-sustaining reaction of the first and second materials produces carbon dioxide gas.
 13. The apparatus of claim 1, where the third material is from the group that comprises ammonia borane, borohydrides and other metal hydrides, carboranes or hydrazines, where the third material when pyrolyzed by the exothermic and self-sustaining reaction of the first and second materials produces hydrogen gas.
 14. Apparatus, comprising: a heat generating structure including a substrate of at least first and second materials formed into a foil structure having at least one layer of the substrate; and a third material located next to or within the heat generating structure, where the first and second materials, upon being thermally energized, react with each other in an exothermic and self-sustaining reaction that propagates from a first location within the structure along a travel path to a second location within the structure and at a rate that depends upon one or more characteristics of the substrate and the coating, and where the exothermic and self-sustaining reaction of the first and second materials pyrolyzes the third material.
 15. A structure, comprising: a substrate comprised of a material arranged in a non-uniform and varying distribution of mass of the substrate material along a length of the substrate in a propagation direction; and a second material located next to or within the substrate material, where the substrate material, upon being thermally energized, reacts in an exothermic and self-sustaining reaction that propagates from a first location within the substrate along a travel path to a second location within the substrate at a rate that depends upon one or more characteristics of the substrate, where a physical configuration of the substrate includes one or more different directions of the substrate material at one or more points along the travel path, and where the exothermic and self-sustaining reaction of the substrate material pyrolyzes the second material.
 16. The structure of claim 15, where the substrate material further comprises an energetic binder from the group that comprises glycidyl azide polymer, polyoxetanes, or polyglycidyl nitrate, and where the substrate material is formed around a wire.
 17. The structure of claim 15, where the substrate material comprises a fluoropolymer from the group that comprises polytetrafluoroethylene, fluoroelastomers, fluorosurfactants, or fluoroadditives, and where the fluoropolymer substrate material contains a plurality of metal particles dispersed therein, where the metal particles are from the group that comprises aluminum, magnesium, boron, beryllium, zirconium, titanium or zinc, and where the substrate material containing the metal particles dispersed therein is extruded to form a filament.
 18. The structure of claim 15, where the substrate material further comprises a binder from the group that comprises hydroxy terminated polybutadiene, hydroxy terminated polyether, carboxy terminated polybutadiene, polyether, polycaprolactone, or polyvinyl chloride.
 19. The structure of claim 17, where the substrate material further comprises a binder from the group that comprises hydroxy terminated polybutadiene, hydroxy terminated polyether, carboxy terminated polybutadiene, polyether, polycaprolactone, and polyvinyl chloride.
 20. The structure of claim 15, where the substrate material comprises a powdered mixture of a fuel material and an oxidizer material, and where the substrate material further comprises an energetic binder from the group that comprises glycidyl azide polymer, polyoxetanes, or polyglycidyl nitrate, and where the substrate material is formed around a wire.
 21. The structure of claim 15, where the substrate material comprises a powdered mixture of a fuel material and an oxidizer material, and where the substrate material further comprises a binder from the group that comprises hydroxy terminated polybutadiene, hydroxy terminated polyether, carboxy terminated polybutadiene, polyether, polycaprolactone, or polyvinyl chloride, and where the substrate material is formed around a wire.
 22. The structure of claim 15, where the substrate comprises a plurality of tubes, where each tube contains the substrate material.
 23. The structure of claim 22, where the substrate material further comprises an energetic binder from the group that comprises glycidyl azide polymer, polyoxetanes, or polyglycidyl nitrate.
 24. The structure of claim 22 where the substrate material comprises a fluoropolymer from the group that comprises polytetrafluoroethylene, fluoroelastomers, fluorosurfactants, or fluoroadditives, and where the fluoropolymer substrate material contains a plurality of metal particles dispersed therein, where the metal particles are from the group that comprises aluminum, magnesium, boron, beryllium, zirconium, titanium or zinc.
 25. The structure of claim 22, where the substrate material further comprises a binder from the group that comprises hydroxy terminated polybutadiene, hydroxy terminated polyether, carboxy terminated polybutadiene, polyether, polycaprolactone, or polyvinyl chloride. 